Effects of Wind Shear and Aerosol Conditions on the Organization of Precipitating Marine Stratocumulus Clouds

This study examines how wind shear affects precipitating marine stratocumulus clouds under different cloud droplet number concentrations (Nd). We performed a series of large eddy simulations (LES) of nocturnal marine stratocumulus clouds using Cloud Model 1 (CM1). The simulations show that Nd is the dominant factor for cloud cellular organization in this cloud regime rather than wind shear. Low Nd characterizes the open cellular structure with a high in-cloud liquid water path (LWP). When wind shear is increased, the cloud fraction tends to decrease along with LWP, suggesting the cloud top region is significantly influenced by the entrainment and mixing of dry air from the free troposphere. We also examine cold pools in open and closed cellular clouds. Open-cell clouds produce larger and deeper cold pools compared to closed-cell clouds. Interestingly, cold pools can exist without surface precipitation and are produced by evaporation of light precipitation (drizzle) below the cloud base with weak downdrafts. The evaporation of raindrops and drizzle play an important role in initiating new convection, particularly where colliding outflows occur downstream of the cloud. This secondary convection contributes to the development and maintenance of the cloud cellular organization and formation. Low altitude clouds cover a wide area of the Earth's atmosphere and play an important climatic role by reflecting sunlight back to space, thereby having been the topic of decades of study via observation and numerical simulation. This study uses high-resolution weather simulations to understand the influence of the winds and tiny particles on subtropical low clouds. We find that the tiny particles largely affect the cellular cloud organization (e.g., broken and unbroken clouds). When the wind near the surface is much weaker than wind above the surface, the cloud amount and thickness are decreased via mixing of the dry air at the cloud top. In addition, the different cloud organizations produced different cold pockets of dense air near the surface. The cold air resulting from broken clouds helps to initiate a new cloud downstream, and convection upstream of the unbroken cloud is enhanced by the cold air outflow. Cloud droplet number concentration dominates vertical shear in determining the cellular cloud organization of idealized marine stratocumulusA stronger wind shear leads to a reduced cloud fraction associated with cloud thinning compared to cases with weaker shearIntermediate Nd simulations with negligible surface precipitation produce cold pools with comparable intensity as a low Nd configuration